• 한국유체기계학회 논문집
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• 제10권2호
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• pp.16-21
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• 2007

Potential advantages of using vapor injection in a two stage rotary compressor for a $CO_2$ heat pump water heater system were addressed in this paper by numerical simulation. Vapor separated from a flash tank in the middle of the expansion process can be used for injection into the second stage suction plenum of the compressor to improve the system performance. Vapor injection increases the intermediate pressure between the two stages, thus increasing the first stage compressor work and reducing that of the second stage. As a whole, however, the compressor input power increases due to injected mass flow rate for the second stage. Computer simulation showed that increment of the cooling capacity by vapor injection exceeded that of the compressor work, thus improving the system performance. COP improvement by vapor injection was calculated to be about 5-14% for normal operating conditions. With vapor injection, a maximum COP was found when the displacement volume of the second stage becomes 90-95% of that of the first stage of the compressor.

• 한국수소및신에너지학회논문집
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• 제22권2호
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• pp.223-231
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• 2011

The thermal efficiency of energy-to-power conversion becomes uneconomically low when the temperature of heat source drops below $370^{\circ}C$. ORC (Organic Rankine Cycle) has attracted much attention in last few years due to its potential in reducing consumption of fossil fuels and relaxing environmental problems, and its favorable characteristics to exploit low-temperature heat sources. In this work thermodynamic performance of ORC using nine working fluids is comparatively assessed. Special attention is paid to the effect of system parameters such as turbine inlet temperature and pressure on the characteristics of the system such as volumetric flow rate and quality at turbine exit, latent heat, net work as well as thermal efficiency. Results show that in selection of working fluid it is required to consider various criteria of performance characteristics as well as the thermal efficiency. Results also show that the system efficiencies become same irrespective of kind of working fluid when the temperature of heat source decreases to low range.

• 한국가시화정보학회지
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• 제9권2호
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• pp.61-65
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• 2011

Several previous works on rocket nozzle flows have revealed the existence of the transition from FSS to RSS and the occurrence of asymmetric flow associated with the boundary layer separation, which can cause excessive side-loads of the propulsion system. Thus, it is of practical importance to investigate the asymmetric flow behaviors of the propulsion nozzle and to develop its control method. In the present study, the asymmetric flow control method using a cavity system was applied to supersonic nozzle flow. Time-dependent asymmetric flow was experimentally investigated with the rate of change of the nozzle pressure ratio. The results obtained showed that the cavity system installed on nozzle wall would be helpful in fixing the unsteady motions of the boundary layer separation, consequently reducing the possibility of the occurrence of the asymmetric flow.

• 상하수도학회지
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• 제27권1호
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• pp.129-138
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• 2013

Water interruption is often caused by a rupture in the branch-like singular pipeline. This will cause critical complaints from household and may decrease public service quality. As an alternative of singular pipeline, additional parallel pipeline could be installed for sustainable water supply. This system is called double pipeline system and able to be utilized for water transmission line between treatment plant and distribution reservoir. Construction of double pipeline was thought to increase capital cost, which can be an issue to waterworks authorities. Reducing capital cost was possible by means of installing connectors between two parallel pipelines because of reduced diameter of each pipe. To obtain optimal design condition for connectors, it was necessary to compare water pressure according to accident location, to investigate flow according to connection pipe spacing, connection pipe diameter, and aging of pipe. Reliable and economical connection layouts were determined based on these results. The cost estimation for each design condition was carried out. Cost was approximately reduced by 20 ~ 30 % compared to the double pipeline without connections. In addition to this, connection between double pipelines could expect extra benefits for maintenance since the pipe could be repaired and rehabilitated without interruption.

• The Journal of Asian Finance, Economics and Business
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• 제8권2호
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• pp.329-338
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• 2021

The mobile payment system has not only changed the payment pattern but also has improved the performance of small and medium-sized enterprises (SMEs) and individuals' quality of life by saving their energy, reducing the risk of taking cash, and enabling them to give more time to businesses and do international business. Besides, with the current advancement in the Internet as well as the digital transformation, SMEs are being forced to adopt mobile payment systems (MPS). The purpose of this study is to investigate the effects of technological-organizational-environmental (TOE) determinants on the adoption of MPS and its impact on SMEs' firm performance operating in Ghana. This research employed a closed-ended questionnaire to collect data from January 2020 - April 2020. This study used a structural equation modeling (SEM) approach using SmartPLS 3.0 for path analysis of 145 responses. The findings of the study include the statistically significant effects of technological (relative advantage and compatibility), organizational (top management support and employees' readiness), and environmental (social influence and competitive pressure) factors on the adoption of MPS. Furthermore, this study also proved the positive and statistically significant effect of MPS on SMEs' performance. This study demonstrated that the TOE factor explains 70% variations in the adoption of MPS.

• Geomechanics and Engineering
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• 제33권2호
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• pp.155-165
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• 2023

Hurricane Katrina swept New Orleans, Louisiana, USA, in 2005, causing more than 1,000 fatalities and severe damage to the flood protection system. Recovery activities are complete, however, clarifying failure mechanisms and devising resilient and cost-effective retrofitting techniques for the flood protection system are still of utmost importance to enhance the general structural integrity of water retaining structures. This study presents extensive centrifuge test results to find various failure mechanisms and effective retrofitting techniques for a levee system. The result confirmed the rotational failure and translational failure mechanisms for the London Ave. Canal levee and 17^th St. Canal levee, respectively. In addition, it found that the floodwalls with fresh waterstop in their joints perform better than those with old/weathered waterstop by decreasing pore water pressure build-up in the levee. Structural caps placed on the top of the joints between I-walls could also prevent local failure by spreading the load to surrounding walls. At the same time, the self-sealing bentonite-sand mixture installed along the riverside of floodwalls could mitigate the failure of floodwalls by blocking the infiltration of seepage water into the gap formed between levee soils and floodwalls.

• 한국음향학회지
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• 제26권3호
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• pp.115-122
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• 2007

본 연구에서는 마이크로스피커의 구조와 특성을 유닛과 덕트형 인클로저로 구성되어 있는 일종의 덕트형 스피커시스템처럼 취급할 수 있음을 처음으로 보여주었다. 후면기공의 면적이 증가할수록 스티프니스는 감소하고 컴플라이언스는 증가하였다. 그 결과로써, 후면기공의 면적이 증가할수록 공명진동수가 컴플라이언스의 제곱근에 비례하여 증가하였다. 후면기공의 면적이 감소함에 의하여 중저음 영역에서의 기준음압레벨이 지수함수적으로 감소하였다.

• 설비공학논문집
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• 제30권1호
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• pp.44-49
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• 2018

High-strength plastic water supply pipe evaluation method was evaluated in this study. Up to date, high strength water supply pipes that we install are mostly ductile cast iron pipes. Sometimes, a few PVC pipes are installed. Metal pipes have rust problem on the surface, causing serious damage to metal pipes and reducing the expected life span of water piping system. Nowadays, depending on technology development, some companies have improved properties of general PVC pipe performance with remarkable properties that exceed KS and ASTM standard. Here, we suggest a new method of performance evaluation for high-strength water plastic pipes.

• 한국소음진동공학회논문집
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• 제13권11호
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• pp.838-844
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• 2003

The noise and vibration reduction schemes of an industrial robot are studied. Experimental procedures are employed to examine the sources of robot noise. A parametric study is undertaken to observe the effects of each part such as gear, shaft and housing on the sound pressure level. After the part which mainly effects on the noise is proved, we propose countermeasures for reducing the noise and vibration of the robot system.

• 한국정밀공학회지
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• 제14권12호
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• pp.95-104
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• 1997

We consider controller design problem to enhance shift quality for automatic transmission. A dynamic modeling related to shifting (mainly 2-3 up-shift) is constructed and nonlinear robust controllers are designed to reduce output torque during shifting. Suggesting a new hydraulic circuit enabling the direct clutch drive, the control activity is extended and more implementable than the conventional design. The designed robust controllers overcome the unmodeled dynamics and the uncertainty embending in the system. Moreover, the dynamic effect between the clutch pressure and the PWM valve duty is considered via singular perturbation technique.